The present invention relates to an optical apparatus and in particular, to an interferometer system for measuring displacements and exposure systems using the same.
Since the use of the Michelson interferometer system at the Michelson-Moley's experiment in 1887 for finding the presence of ether, interferometers have been widely used in various technical fields requiring precise measurement. Interferometers have been broadly employed in the fields of fine industries including operating works of exposure, diamond turning, precision processing works, and so forth, because they are capable of measuring a target with the accuracy at the level of 1 nm. Especially, since the development of the laser in 1960, a laser interferometer using a laser as a light source thereof contributes to enhancing the accuracy of measurement over a variety of industrial fields. The interferometer is an optical device using the physical phenomenon of interference of light that is induced when an optical path length of a measuring beam passing through a target object is different from a fixed optical path length of a reference beam.
FIG. 1 is a systemic diagram showing a basic structure of a Michelson interferometer. As illustrated by FIG. 1, a beam of light emitted from a light source S is divided into a reference beam RB and a measuring beam MB by a beam splitter BS, each progressing toward a reference mirror M1 and a moving mirror M2. The reference and measuring beams, RB and MB, are reflected on the reference and moving mirrors, M1 and M2, respectively, and then all return to the beam splitter BS. Thereafter, the reference beam RB is partially incident on a detector D by way of the beam splitter BS, while the measuring beam MB is partially incident on the detector D after being reflected on the beam splitter BS. The reference and measuring beams, RB and MB, incident on the detector D are superimposed to each other to form an interference fringe.
As well known, such an interference effect may be summarized by Equation 1 as follows. In Equation 1, the parameters I, I1, and I2 denote intensities of the interference fringe, the reference beam RB, and the measuring beam MB, respectively, and the parameter δ denotes a phase difference between the reference beam RB and the measuring beam MB.I=I1+I2+2√{square root over (I1I2 )}cos δ  (1)
The variation in the intensity of the interference fringe is induced by the phase difference δ. Therefore, if it determines the number of migrations of the interference fringe monitored at the detector D, a position of the moving mirror M2 may be obtained by Equation 2 as follows.
                    X        =                              X            0                    +                      N            ⁢                          λ              2                                                          (        2        )            In Equation 2, the parameters X, X0, N, and λ represent a displacement of the moving mirror M2, an initial position of the moving mirror M2, the number of interference fringes, and a wavelength of the lightwave used, respectively.
FIG. 2 is a perspective diagram showing a typical X-Y stage system and a displacement interferometer for determining a position of the stage system. As illustrated in FIG. 2, the typical X-Y stage system 10 is constructed with including a fixed stage base 12, a lower stage 14 disposed on the stage base 12, and a higher stage 16 disposed on the lower stage 14. The lower stage 14 is movable along the x-direction to the stage base 12, while the higher stage 16 is movable along the y-direction to the stage base 12. Thus, the higher stage 16 is able to move in second dimensions along both the x- and y-directions to the stage base 12.
Around the X-Y stage system 10, an optical interference system is arranged to measure x- and y-positions, and yaw (left and right trembling), of the higher stage 16. The optical interference system includes a light source 50 emitting a laser beam of a predetermined wavelength, beam splitters 1˜4 distributing the laser beam 55 emitted from the light source 50, and interferometers, 20, 30, and 36, each measuring x and y positions, and yaw, of the higher stage 16, by means of the laser beams 55 divided by the beam splitters 1˜4. In addition to the peripheral of the X-Y stage system 10, a wavelength tracker 40 may be disposed to measure a refractive index of air on purpose to monitor environmental variations such as temperature and pressure.
The x-interferometer 20 is composed of an x-measuring mirror 21, an x-beam splitter 22, and an x-detector 23, while the y-interferometer 30 is composed of a y-measuring mirror 31, a y-beam splitter 32, and a y-detector 33. The yaw interferometer 36 is constructed of the y-measuring mirror 31, a yaw-beam splitter 34, and a yaw-detector 35. The x- and y-measuring mirrors, 21 and 31, are adhesive to sidewalls of the higher stage 16 in order to create optical path differences in accordance with positional variations of the higher stage 16, and are oriented in parallel with the x- and y-directions. Further, the x-, y-, and yaw-beam splitters, 22, 32, and 34, are each comprised of reference mirrors to form their own reference beams.
Other components of the optical interference system, besides the x- and y-measuring mirrors 21 and 31, are fixed to the stage base 12. Therefore, the higher stage 16 is able to move in second dimensions to the light source 50. Meanwhile, in order to measure a relative motion of the higher stage 16, the laser beam 55 incident on the x- and y-measuring mirrors 21 and 31 needs to be reflected thereon in a normal direction. For that purpose, the measuring mirrors 21 and 31 should be manufactured in a size capable of assuring such a normal reflection. If the x-measuring mirror 21 is too small, an x-measuring beam 55 would stray from the x-measuring mirror 21 by a y-directional motion of the higher stage 16. To prevent the deviation of the beam, it is required for the x-measuring mirror 21 to be designed with a size larger than the maximum displacement along the y-direction of the higher stage 16 to the x-beam splitter 22. The requirement for the size of the measuring mirror is also applicable to the y-measuring mirror 31.
It is quite general to fabricate a stage, which is used in the field of fine industries such as semiconductor manufacturing processing in particular, to be operable with a very high accuracy of motion. In accomplishing the highly accurate control facility, the measuring mirrors for determining displacements must be also designed and manufactured with very high uniformities. Specifically, an exposure system is required to be operable with the uniformity at the level of several nanometers because surface uniformities of the measuring mirrors directly affect distortions and overlays of patterns transcribed to wafers.
However, as aforementioned, although there is a need for manufacturing the measuring mirror (either the x-measuring mirror or the y-measuring mirror) in a size capable of preventing the deviation of a measuring beam, it is very difficult and incurs significant cost in manufacturing the measuring mirror with such high uniformity at the level of several nanometers. Moreover, since the surface uniformity of the measuring mirror may be reduced in accordance with gravity, temperature variation, and acceleration by motion, continuous maintenance is required for keeping the uniformity of the measuring mirrors on the same level. This high level of maintenance can result in significant maintenance expenses.